In the replacement of a knee by an artificial joint, it is necessary to shape the distal end of the femur by making a number of cuts through the condyles to precisely receive the femoral component of the replacement joint. The femoral component is of hollow bowl or generally "C" shape, and it has a generally concave inner surface designed to fit with the cut end of the femur. The cuts which are made may need some slight adjustments in order to ensure that the implant will ultimately fit very closely over the resected bone with what is known as an interference fit, that is, a tight fit such that the implant could be, for example, up to 2% smaller than the bone to which it is to be fitted. In this way the implant will be a tight fit on the bone and there will be no chance of the implant rocking on the bone and so leading to failure. Also any gaps or deviations greater than this are counterproductive with respect to bone incorporation into the surfaces of the implant.
When preparing the femur to receive its respective component of the artificial knee joint, the various cuts are made using a guide which should result in a final reasonably accurate shape. See, for example, the guide described in co-assigned British Patent Application No. 9213766.0. In particular, those condylar cuts which are across the axis of the bone and which take most of the load, do not contribute much to the interference fit of the implant and are angled to optimize load distribution between the bone and the implant whilst also ensuring bone incorporation. However, those condylar cuts which run parallel or close to parallel to the axis of the bone do need further preparation to ensure the final accurate interference fit and ideally this is achieved by making the appropriate cuts such that the bone is very slightly over size and then subjecting it to a final rasping action to achieve the exact size required.
Once the cuts have been made it is usual for a femoral trial, which is of the same shape and size as the final knee implant, to be fitted over the resected femur to check the correct sizing and positioning of the cuts. Thereafter once the trial has been removed, the final implant has to be manoeuvred into place taking care not to substantially alter the cut surfaces of the bone or damage the articulation surface of the artificial joint.
For these reasons it is desirable to have some sort of impactor/extractor holder which can grasp the trial and the final implant and help one in positioning them on the cut end of the femur. The holder must be substantial, since it will often be necessary to hammer it to force both the trial and the implant into final position. Equally however the holder must not damage the highly polished articulation surface of the final implant. Further, there must be very secure attachment between the holder and either the implant or the trial, so that they become, in effect, one and the same whilst the trial and/or implant are being manoeuvred into position on the cut femur.
U.S. Pat. No. 5,059,196 (Coates) shows a femoral prosthesis holder/driver tool and method of implantation using same. That tool includes impact pads, and fingers that are spread to grip the internal wall adjacent the intercondylar notch opening. That patent asserts an advantage to having fingers which grip the internal walls of the prosthesis as being "minimal risk of trauma to the underlying tissue of the joint capsule, from contact with various components of the tool." It is, however, believed that with such internal fingers the posterior cruciate ligament may interfere with placement of the prosthesis.